March 19, 2014

Exploring the Transcend Wifi-SD card

There are a number variations on a saying ”Never let a programmer have a soldering iron; and never let an engineer have a compiler”

It’s been my experience over many years that hardware people are responsible for rubbish software. Years upon years of shoddy hardware drivers, dreadful software bundled with cameras (Canon, Pentax I’m looking at both of you); Printers (HP, Epson), Scanners (HP – one day I might forgive you) have provided the evidence. Since leaving Microsoft I’ve spent more time working with Linux, and every so often I get into a rant about the lack of quality control: not going back and fixing bugs, not writing proper documentation (the “Who needs documentation when you’ve got Google” attitude meant when working on one client problem all we could find told us it could not be solved. Only a lucky accident found the solution). Anyone can program: my frustrations arise when they do it without proper specification, testing regime, documentation and “after care”. The Question is … what happens when Engineers botch together an embedded Linux system.

Let me introduce you to what I believe to be the smallest commercially available Linux computer and Web server.

I’ve bought this in its Transcend form – which is available for about £25. It’s a 16GB memory card, an ARM processor and a WIFI chip all in an SD card package. Of course chip designers will be able to make it smaller but since it’s already too easy to lose a Micro-SD card, I’m not sure the would be any point in squeezing it into a smaller form factor. Transcend aren’t the only firm to use the same hardware. There is a page on OpenWrt.Org which shows that Trek’s Flu-Card, and PQI’s Aircard use the same hardware and core software. The Flu card is of particular interest to me, as Pentax have just released the O-FC1 : a custom version of the flu card with additional functions including the ability to remotely control their new K3 DSLR. Since I don’t have the K3 (yet) and Pentax card is fairly expensive I went for the cheap generic option.

The way these cards works is different from the better known Eye-FI card. They are SERVERS : they don’t upload pictures to a service by themselves, instead they expect a client to come to them, discover the files they want and download them. The way we’re expected to do this is using HTTP , either from a web browser or from an App on a mobile device which acts as wrapper for the same HTTP requests. If you want your pictures to be uploaded to photo sharing sites like flickr, photobucket, smugmug, one line storage like Dropbox, Onedrive (nee skydrive), or social media sites (take your pick) these cards – as shipped – won’t do that. Personally I don’t want that, so that limitation’s fine. The cards default to being an access point on their own network – which is known as “Direct share mode” – it feels odd but can be changed.

Various people have reported that Wifi functionality doesn’t start if you plug the card into the SD slot of a laptop; and it’s suggested this is a function of the power supplied. Transcend supply a USB card reader in the package, and plugged into it my brand-new card soon popped up as a new wireless network. It’s not instant – there’s an OS to load – but it’s less than a minute. This has another point for use in a camera: if the camera powers down, the network goes with it; so the camera has to stay on long enough for you to complete whatever operations are needed.

The new card names its network WIFISD and switching to that – which has a default Key of 12345678 – gave me wireless connection with a nominal connection speed of 72Mbits/sec and a new IP configuration, a Connection-Specific DNS Suffix of WifiCard, an IP Address of 192.168.11.11 and DNS server, Default gateway, and DHCP server of 192.168.11.254 : that’s the server. The first thing I did to point my browser at 192.168.11.254, enter the login details (user admin, password admin) and hey presto up came the home page. This looks like it was designed by someone with the graphic design skills of a hardware engineer, or possibly a blind person. I mean, I know the card is cheap, but effort seemed to have gone in to making it look cheap AND nasty.

However with the [F12] developer tools toggled on in Internet explorer I get to my favourite tool. Network monitor. First of all I get a list of what has been fetched, and if I look at Details for one of the requests, the response headers tell me the clock was set to 01 Jan 2012 when the system started and the server is Boa/0.94.14rc21

The main page has 4 other pages which are arranged as a set of frames. frame1 is the menu on the left, frame2 is the banner (it only contains Banner.jpg) and frame3 initally holds page.html, which just contains page.jpg and there is a blank.html to help the layout. Everything of interest is in frame1, what is interesting is that you can navigate to frame1.html without entering a user name and password and from there you can click settings and reset the admin password. The settings page is built by a perl script (/cgi-bin/kcard_edit_config_insup.pl) and if you view the page source, the administrator password is there in the html form so you don’t even need to reset it. Secure ? Not a bit of it. Within 5 minutes of plugging the card in I’d found a security loophole (I was aware of others before I started thanks to the openwrt page and Pablo’s investigation). I love the way that Linux fans tell me you can build secure systems with Linux (true) and it can be used on tiny bits of embedded hardware where Windows just isn’t a an option (obviously true here): but you don’t automatically get both at the same time. A system is only as good as the people who specified, tested, documented and patched it.

While I had the settings page open I set the card to work in “internet mode” by default and gave it the details of my access point. You can specify 3 access points; it seems if the card can’t find a known access point it drops back to direct share mode so you can get in and change the settings (I haven’t tried this for myself). So now the card is on my home wifi network with an address from that network. (The card does nothing to tell you the address, so you have to discover it for yourself). Since there is a just a process of trying to connect to an access point with a pre-shared key, any hotspots which need a browser-based sign-on won’t work.

The next step was to start exploring the File / Photo / Video pages. Using the same IE monitor as before it’s quite easy to see how they work – although Files is a Perl script and pictures & videos are .cgi files the result is the same. A page which calls /cgi-bin/tslist?PATH=%2Fwww%2Fsd%2FDCIM and processes the results. What’s interesting is that path /www/sd/DCIM. It looks like an absolute path… What is returned by changing to path to, for example, / ? A quick test showed that /cgi-bin/tslist?PATH=%2F does return the contents of the root directory. So /cgi-bin/tslist?PATH={whatever}requires no security and shows the contents of any directory.The pictures page shows additional calls to /cgi-bin/tscmd?CMD=GET_EXIF&PIC={fullpath} and /cgi-bin/thumbNail?fn={full path}. The files page makes calls to /cgi-bin/tscmd?CMD=GET_FILE_INFO&FILE={full path} (picture EXIF is a bit disappointing it doesn’t show Lens, or shutter settings, or camera model or exposure time it just shows file size – at least with files we see modified date; thumbnail is also a disappointment. There is a copy of DCRAW included on the system which is quite capable of extracting the thumbnail stored in the raw files, but it’s not used) And there is a link to download the files /cgi-bin/wifi_download?fn={name}fd={directory}. By the way, notice the lack of consistency of parameter naming the same role is filled by PATH=, PIC=, fn= and fn=&fd= was there an organised specification for this ?

Shock horror! More sloppiness in the CGI scripts, the last response header is followed not by [CR][LF] but by [LF][LF] fortunately Lee Holmes has already got an answer for this one. I also found found the space in my folder path /www/sd/hello James caused a problem. When it ran through [System.Web.HttpUtility]::UrlEncode the space became a + sign not the %20 in the line above: the CGI only accepts %20, so that needs to be fixed up. Grrr.

Since we can get access to any of the files on the server we can examine all the configuration files, and those which control the start-up are of particular interest. Pablo’s post was the first that I saw where someone had spotted that init looks for a autorun.sh script in the root of the SD file system which can start services which aren’t normally launched. There seems to be only one method quoted for starting an FTP service tcpsvd -E 0.0.0.0 21 ftpd -w / &There are more ways given for starting the telnet service, and it looks for all the world as if this revision of transcend card has a non-working version of telnetd (a lot of the utilities are in a single busybox executable), so Pablo resorted to getting a complete busybox, quickly installing it and using cp /mnt/sd/busybox-armv5l /sbin/busybox chmod a+x /sbin/busybox /sbin/busybox telnetd -l /bin/bash &

This was the only one which worked for me. Neither ftp nor telnet need any credentials: with Telnet access it doesn’t take long to find the Linux Kernel is 2.6.32.28, the Wi-Fi is an Atheros AR6003 11n and the package is a KeyASIC WIFI-SD (searching for this turns up pages by people who have already been down this track), or more specifically KeyASIC Ka2000 EVM with an ARM926EJ-S CPU, which seems to be used in tablets as well.

Poking around inside the system there are various references to “Instant upload” and to G-PLUS but there doesn’t seem to be anything present to upload to any of the services I talked about before, when shooting gigabytes of photos it doesn’t really make sense to send them up to the cloud before reviewing and editing them. In fact even my one-generation-behind camera creates problems of data volume. File transfer with FTP is faster than HTTP but it is still slow. HTTP manages about 500KBytes/sec and FTP between 750 and 900KBytes/Sec. That’s just too slow, much too slow. Looking at some recent studio shoots I’ve use 8GB of storage in 2 hours: averaging a bit more than 1MB/Second.With my K5, RAW files are roughly 22MB so take about 45 seconds to transfer using HTTP but it can shoot 7 frames in a second – and then spend five minutes to transferring the files: it’s quicker to take the memory card out of the camera, plug it into the computer, copy files and return the card to the camera. It might get away with light use, shooting JPGs, but in those situations – which usually mean wandering round snapping a picture here and a picture there – would your WiFi connected machine be setup and in range ?

The sweet spot seems to be running something on a laptop / tablet phone to transfer preview JPGs – using lower than maximum resolution, and some compression rather than best quality (the worry here is forgetting to go back to best possible JPEG and turning RAW support off). In this situation it really is a moot point which end is the client and which end is the server. Having the card upload every file to the cloud is going run into problems with the volume of data, connecting to access points and so on. So is pulling any great number of RAW files off the card. Writing apps to do this might be fun, and of course there’s a world of possible hacks for the card itself.